1. Field of the Invention
This invention involves ZnCl.sub.2 fibers for transmission of electromagnetic energy in the infrared portion of the spectrum.
2. Description of the Prior Art
Despite the fact that glass fibers have been known to transmit light for centuries, the application of such fibers to the long-distance transmission of information has been earnestly pursued for only approximately the past decade. In addition to the relatively recent availability of appropriate sources and detectors, this development has been stimulated by the realization that such fibers may be fabricated with low loss (less than 20 dB/km). Such low losses are necessary if information is to be transmitted over significant distances with a commercially viable repeater spacing.
It is widely recognized that the current interest in optical fibers was spurred in part by the realization in the mid-1960's that certain simple glasses have inherent losses between 1 and 10 dB/km, and that the then high-loss observations in simple glasses were due almost entirely to impurities which might be removed with careful processing. Glass fabrication techniques which have been developed over the past 10 years, including the CVD, MCVD and soot techniques, have yielded fibers with losses as low as 0.5 dB/km. In addition, techniques for the fabrication of graded fibers enable the realization of low loss characteristics even when multimode transmission configurations are utilized.
Present developmental emphasis centers mainly on fiber loss characteristics in the near region of the optical spectrum, i.e., 0.7 to 2 microns. This emphasis is due in part to the readily available detectors and sources which may be efficiently operated in this region of the spectrum. However, practical and theoretical considerations which govern transmission in optical waveguides suggest only minimal distinction between the visible and infrared regions of the spectrum, and consequently viable commercial alternatives in the infrared portion of the spectrum would have few inherent shortcomings.
While complex glasses were originally considered in the early development of optical fibers, such configurations have been largely foresaken in preference to the simpler silica and doped silica compounds. Nevertheless, materials, other than the now standard silica based compounds, may be found to be useful in particular regions of the spectrum. Such materials, many of which are known to exist in the glass state, may be found to be particularly beneficial for specific applications.
ZnCl.sub.2, while it has been known to exist in the glass state (see, for example, Naturwissenschaften Vol. 44, p. 536 [1957]), has been relatively difficult to fabricate and has certain inherently poor physical characteristics. As a result little thought has been given to the use of ZnCl.sub.2 in optical fibers and no suggestion has been made that it might possess certain outstanding transmission characteristics.